1. Field of the Invention
The present invention generally relates to a bias voltage controlling apparatus, an information reproducing apparatus and an information recording apparatus including such a bias voltage controlling apparatus. More particularly, the present invention relates to a bias voltage controlling apparatus for controlling a bias voltage in a focus servo control for controlling a focal point of a light beam when optically recording or reproducing information with respect to an information record surface of an information record medium, and an information reproducing apparatus and an information recording apparatus including such a bias voltage controlling apparatus.
2. Description of the Related Art
When optically recording or reproducing information with respect to an information record medium such as an optical disc, it is necessary to make a focal point of a light beam, such as a laser light for recording or reproducing the information, accurately coincident with a position on an information track of an information record surface of the information record medium.
At this time, as a positional control of the focal point, there are a positional control in a direction perpendicular to the information record surface and a positional control in a direction parallel to the information record surface. Among them, the positional control in the direction perpendicular to the information record surface is often performed by a so-called focus servo control.
Here, as a method of performing the focus servo control, there is a so-called astigmatic method or a so-called Foucault method. In either of these methods, a so-called S curve as shown in FIG. 14 is obtained as a focus error signal S.sub.FE on the basis of a reflection light of a light beam from an information record medium, and the position of the focal point of the light beam is made coincident with the position of the information record surface by moving an objective lens in a direction perpendicular to the information record surface by an actuator etc., so that the focus error signal S.sub.FE becomes "0" when a servo loop of a focus servo for this S curve (i.e., the focus error signal S.sub.FE) is in a servo close condition.
At this time, the S curve shown in FIG. 14 is obtained as follows. Namely, in case of the focus servo control by means of the astigmatic method for example, the reflection light of the light beam, to which an astigmatism is given, is received or detected by a four divided light detector. Then, a difference between (i) a signal obtained by adding detection signals of two light-detecting portions on one diagonal line of the four divided light detector and (ii) a signal obtained by adding detection signals of two light-detecting portions on another diagonal line of the four divided light detector is calculated. Such an S curve calculated as the difference has an output voltage corresponding to a focus error amount of the focal position with respect to the information record surface.
By the way, in an optical disc reproducing apparatus or an optical disc recording apparatus (which is simply referred to as an "optical disc reproducing apparatus etc.," hereinafter), there may be such a case that the output of the focus error signal S.sub.FE does not become "0" although the focal point of the light beam and the information record surface are coincident with each other as shown in an upper half portion of FIG. 14 (i.e., although it corresponds to a point there the error amount is "0" on the graph shown in the upper half portion of FIG. 14), due to an adjustment drift at the initial time (at the time of manufacturing) with respect to the optical system (e.g., a light detector, a beam splitter, a 1/4.lambda. plate and the like) included in the optical disc reproducing apparatus etc., and/or due to the difference in the sensitivity between the 4 light receiving elements of the 4 divided type light detector (although the sensitivities thereof are originally the same to each other but may be different from each other later due to an aged deterioration or the like).
Here, the adjustment at the initial time with respect to the optical system is to adjust the optical axis, the angle or the like of each constitutional elements of the optical system so that a reflection light of a light beam from the optical disc becomes circle on the light receiving surface of the light detector for receiving the reflection light when the focal point of the light beam is positioned on the information record surface, at the time of manufacturing the apparatus. Then, in the actual optical disc reproducing apparatus etc., since there is a certain limit of the accuracy in the adjustment for the optical system, there may be a case that the output of the focus error signal S.sub.FE does not become "0" although the focal point of the light beam is positioned on the information record surface (i.e., the level of the RF (Radio Frequency) signal S.sub.RF which is the output signal of the light detector is the maximum in a lower half portion of FIG. 14).
In addition, such a phenomenon that the output of the focus error signal S.sub.FE does not become "0" although the focal point of the light beam is positioned on the information record surface may appear outstandingly in a CD (Compact Disc) player of on-vehicle-type. In this case, since an accurate focus error signal S.sub.FE corresponding to the RF signal S.sub.RF cannot be obtained, it is difficult or impossible to perform an accurate focus servo control, resulting in the degradation of the S/N (Signal/Noise) ratio. Thus, it is difficult or impossible to perform an accurate information reproduction.
Therefore, in case that the shift of the S curve is generated as shown in the upper half portion of FIG. 14 (the drift between the focal point and the position of the objective lens where the level of the S curve becomes "0" is generated), it is important to superimpose an appropriate bias voltage onto the S curve so that the focal point (the position where the level of the RF signal S.sub.RF becomes the maximum) and the position of the objective lens where the level of the S curve becomes "0" are coincident with each other.
Then, as a method of detecting this appropriate bias voltage, there is a method of detecting a bias voltage at which the voltage level of the RF signal S.sub.RF becomes the maximum, and then setting this detected bias voltage as an appropriate bias voltage, to thereby perform the focus servo control by using this set bias voltage.
On the other hand, there is a so called MD (Mini Disc) as a small-sized and light-weighted optical disc. As such a MD, there are a so called pre-mastered MD (Music MD) exclusive for reproducing and a so called recordable MD capable of recording for a user.
However, in case of reproducing the pre-mastered MD, if the bias voltage is set so that the level of the RF signal S.sub.RF becomes the maximum, the change on the time axis of the component included in the RF signal S.sub.RF (hereinafter, it is referred to as "jitter on the time axis" or simply "jitter") becomes the minimum, and further the allowable width of the change of the positional relationship between the optical elements in the apparatus due to the aged deterioration and the allowable width of the aged deterioration of the bias voltage which is set once in advance (those allowable width will be collectively called as "mechanical allowable width") becomes the maximum. On the other hand, in case of reproducing the information recorded on the recordable MD, if the bias voltage is set so that the level of the RF signal S.sub.RF becomes the maximum, the jitter included in the RF signal S.sub.RF is increased and the mechanical allowable width is decreased, which is a problem.
The increase of the jitter in the RF signal S.sub.RF means that, if a binary coding is performed by a threshold judgment to reproduce the recorded digital value from the RF signal S.sub.RF, the change on the time axis of the rising up timing or falling down timing of the RF signal S.sub.RF is increased, resulting in such a problem that the digital value cannot be correctly reproduced.
Further, this problem due to the jitter becomes more serious as the change due to the aged deterioration increases than the problem caused by the fact that the level of the RF signal S.sub.RF does not become the maximum, to thereby bring about an erroneous detection of the digital value.